This invention relates to a method for controlling a refractive index of a dry plating film wherein a thin film having a desired refractive index can be reliably made according to a dry plating process. The invention also relates to a method for making a dry plating built-up film wherein thin films having different refractive indices are built up along the thickness thereof to provide a built-up film suited for use as an optical filter or the like.
Where a desired refractive index is obtained, usual practice is to use an oxide film. However, with an oxide film, a refractive index does not change significantly by changing a ratio of oxygen in the oxide film. This places a limit on the value of refractive index. In addition, it is difficult to obtain a built-up film whose refractive index is arbitrarily changed as required.
A first object of the invention is to provide a method for controlling a refractive index of a dry plating film wherein a thin film having a refractive index ranging from 1.4 to 2.8 can be reliably made according to a dry plating process.
A second object of the invention is to provide a method for simply, reliably making a dry plating built-up film whose refractive index can be arbitrarily changed along its thickness.
The inventors made intensive studies in order to achieve the above objects, and as a result, found that when using silicon carbide (SiC) as a target in a sputtering process or as an evaporation source in a vacuum evaporation process and changing the concentration of a reactive gas such as oxygen gas or nitrogen gas, there can be formed a thin film made primarily of SiC wherein the content of carbon is controlled to provide an arbitrary refractive index in the range of 1.4 to 2.8 (on measurement at 25xc2x0 C.) depending on the concentration.
According to one embodiment of the invention, there is provided a method for controlling a refractive index of a dry plating film which comprises providing silicon carbide as a starting source and subjecting the silicon carbide to dry plating while controlling a concentration of a reactive gas, thereby forming a thin film, on a substrate, made primarily of silicon carbide and having a refractive index ranging from 1.4 to 2.8.
The inventors also found that when using silicon carbide (SiC) as a target in a sputtering process and changing making electric power against the target, a thin film having an arbitrary refractive index in the range of 1.4 to 2.8 (on measurement at 25xc2x0 C.) depending on the making electric power can be formed. In this case, when sputtering is carried out while changing the concentration of a reactive gas, such as oxygen or nitrogen gas, there can be more effectively formed a thin film made primarily of SiC and having an arbitrary refractive index.
Thus, according to the second embodiment of the invention, there is provided a method for controlling a refractive index of a sputter film, which comprises providing silicon carbide as a target, and subjecting the silicon carbide to sputtering while controlling electric power charged against a target, thereby forming, on a substrate, a thin film made primarily of silicon carbide which has a refractive index ranging from 1.4 to 2.8.
It has also been found that when dry plating is effected in the first embodiment while changing the concentration of the reactive gas continuously or intermittently, the resultant dry plating film has a refractive index which is changed in the form of a wave form, such as a rectangular wave, triangular form, sine wave form or the like, along the thickness thereof. Likewise, when sputtering is effected in the second embodiment while changing the electric power against the target continuously or intermittently and, optionally, changing the concentration of a reactive gas continuously or intermittently, the resultant sputtering film has a refractive index which changes in a wave form such as a rectangular, triangular, sine wave or the like form along the thickness thereof. If the film is used as a filter capable of passing an arbitrary band, a built-up film, which is useful as an anti-reflective film such as a visible light anti-reflective film or the like, can be simply formed.
Hence, according to a third embodiment of the invention, there is provided a method for making a dry plating built-up film, which comprises providing silicon carbide as a starting source, and subjecting the silicon carbide to dry plating while changing a concentration of a reactive gas continuously or intermittently, thereby depositing and forming a thin film having different refractive indices along its thickness.
According to a fourth embodiment of the invention, there is provided a method for making a sputter built-up film which comprises providing silicon carbide as a target, and subjecting the silicon carbide to sputtering while changing making electric power against the target continuously or intermittently, thereby depositing and forming a thin film having different refractive indices along its thickness.